The present invention relates to a color cathode ray tube having in an envelope including a faceplate, a cone and a neck. In line electron beam producing means disposed within the neck produce three electron beams which lie, when undeflected, substantially in a single plane on which plane the longitudinal axis lies, the axis of the central beam, when undeflected, substantially coinciding with the longitudinal axis. A cathodoluminescent screen is provided on the faceplate and a shadowmask lies adjacent to but spaced from, the faceplate. A deflection unit includes a field deflection coil and a line deflection coil. Magnetic field conducting means are arranged such that when the field deflection coil is energized, the magnetic field deflection field produced at the central deflection area is barrel shaped.
U.S. Pat. No. 4,242,612 discloses a deflection unit comprising a line deflection coil which is constituted by two diametrically oppositely arranged coil portions for deflecting an electron beam in a first, horizontal direction, a field deflection coil which is constituted by two diametrically oppositely arranged field coil portions for deflecting an electron beam in a second, vertical direction transverse to the first direction, and magnetic field conducting means comprising two soft-magnetic plates located diametrically opposite to each other between the field and line deflection coils adjacent the center of the field deflection coil. The purpose of these plates is to provide field astigmatism correction by adjusting the magnetic field produced when the deflection unit is energized in such a manner that it is barrel shaped at the central deflection area. In the known deflection coil unit the soft-magnetic plates are located radially outwards of the line deflection coil so that the plates have only a slight influence, if any, on the line deflection magnetic field.
The soft magnetic plates are normally held captive within the thickness of a resin shell inside which the line deflection coil, which may comprise two saddle-type coil portions, is arranged. If the field deflection coil comprises two saddle-type portions then they are mounted externally of the resin shell, rotated 90.degree. relative to the line coil portions, and an annular core member of soft magnetic material, otherwise termed the yoke ring, encloses the field deflection coil. Alternatively if the field deflection coil comprises two toroidally wound coil portions then these are wound on the annular core member of soft magnetic material, which member encloses the resin shell.
While such a deflection unit functions satisfactorily, there is always a requirement by set makers to improve the sensitivity of the deflection unit while minimizing coma, raster and astigmatic effects.